This is a National Priorities List Superfund Site. Tailings fluid seepage contaminated the groundwater in three separate hydrostratigraphic units: one alluvium unit and two gently inclined bedrock units. The three water-bearing units comprise a hydrogeologically complex site with each unit having distinctive geochemistry and groundwater flow directions. The monitoring well network covers 8,000 feet and our database of groundwater elevations and chemical analyses covers 22 years. One of the three units is the subject of ongoing active remediation because the advancing contaminant plume remains geochemically aggressive. A key objective is to arrest this plume advance before it crosses the site property boundary. Within another hydrostratigraphic unit extraction pumping was stopped in 2001 to conduct a natural attenuation test – at the request of the agencies, this test continues to present. Stakeholders include the client, EPA, NRC, New Mexico Environment Department, and the adjacent Indian Nation EPA.
Significant challenges to the project included:
- The presence of three separate hydrostratigraphic units has allowed the development of three separate groundwater contamination plumes – the site hydrogeology and geochemistry are complex.
- Dealing with sometimes varying groundwater cleanup standards tied to three regulatory agencies: EPA, NRC, and New Mexico Environment Department.
- EPA’s requirement that the client develop a new, Site-Wide Supplemental Feasibility study is unusual and challenging – usually the EPA’s original Feasibility Study is a permanent benchmark from which the key results are included in the Record of Decision (ROD).
- The lower sandstone hydrostratigraphic unit has a geochemically aggressive plume that is advancing toward the site boundary – extraction well pumping continues but the wells have short lifespans due to limited saturated thickness.
- The alluvium hydrostratigraphic unit has a plume best characterized as a migrating alkalinity front, which has a tendency to mobilize adsorbed uranium on the mineral grains back to the aqueous phase. However, the adsorbed uranium is a background condition that is unrelated to the tailings fluid impact.
- The ROD-designated background water has been derived from upgradient surface discharges of deep mine waters – prior to these discharges, the near-site bedrock and alluvium were largely unsaturated. For multiple chemical constituents in the groundwater, the statistically derived background water concentrations exceed the site cleanup standards, rendering attainment of the standards impossible without complete dewatering .
- Bringing the site to corrective-action closure so that it can be turned over to the Department of Energy for long-term stewardship monitoring, including turnover of the site’s NRC Source Materials License to DOE.
The Interdisciplinary Approach
Extensive initial effort was given to review of the very large body of data, information, and reports spanning over 30 years. Our Equis environmental database allows relational queries to quickly generate useful output, e.g., time-series charts for any constituents at any monitoring well. Generation of Stiff geochemical diagrams aided early classification of the various types of background and contaminated waters, and they show the chemical evolution of the contaminated waters. Our proprietary software facilitates 3D visualization graphics. Analytical models of groundwater flow and mass flux were developed to support the design and testing of extraction and injection well arrays. Development of annual reports on Groundwater Corrective Action involves detailed analysis of all chemical constituents in all three hydrostratigraphic units. Advanced statistical analyses have (1) defined background water concentrations of constituents in all three units, (2) fostered submittal of Alternate Concentration Limits to revise some NRC cleanup standards, and (3) defined Exposure Point Concentrations toward development of a new site risk assessment, which Chester is currently preparing. With numerous agency stakeholders the fundamental requirement for project management is effective communication, communication, communication!
The Sustainable Result
- Fostering a collaborative relationship with the multiple agency stakeholders.
- Successful applications to change the NRC License groundwater protection standards for combined radium and chloroform. NRC review of an additional Alternate Concentration Limits application is pending.
- Detailed technical hydrogeologic analysis of the lower sandstone unit has fostered development of the current remedial program (extraction well pumping with transfer of the water to an evaporation pond). We are currently implementing an additional remedial enhancement in this same area, comprising injection wells for alkalinity-amended water to lower the pH and geochemically stabilize the water.
- Two of the three hydrostratigraphic units are in full compliance with the NRC groundwater protection standards.
- Development of a Site-Wide Supplemental Feasibility Study places the client in an inherently advantageous position (this is a very substantial, multiyear task).
- Proposed to the agencies specific plans to achieve site closure, including: Institutional Controls and related administrative actions; monitored natural attenuation; technical impracticability waivers for specific chemicals; alternate concentration limits applications for specific chemicals; ROD amendments for specific chemicals; and arresting advance of the aggressive plume in the lower sandstone unit.